Sweetie, it all boils down to this.
As firm as YOUR convictions are about premarital sex, you cannot control the choices that your friend makes.
Not even the choices of your BEST friend who says she is a Christian.
I do not believe in having an "intervention" with your parents and her parents all together.
Tell her parents in private, if you absolutely feel that the Lord is leading you too, that they need to talk to their daughter about her physical relationship that is developing between her and her boyfriend.
Mention it once. Mention it seriously. Mention is briefly.
Then move on. Be a prayer warrior for her in a mighty way. Tell her periodically that you haven't forgotten about praying for her.
Other than that, you can't control her choices here.
You said that the things that she says to you shocks you. Even best friends will to that to each other on purpose. She may do that to you because she feels you are trying to control her or judge her.
This is a hard place to be, I know. Having your best friend make choices that you KNOW are not good for him or her.
But trust me, your faithful prayers and a "brief" word to her parents in private will go a LOT further than a group intervention.
If there is to be an intervention, it need to come from her parents, namely her mother, in the privacy of their home.
Love your friend whether she makes the right choice or not. And pray for her everyday.